1. Field of the Invention
The present invention relates to a switching power supply system that drives a switching device connected in series to the primary winding of an insulating transformer to obtain a specified output voltage from the secondary winding of the insulating transformer, and particularly to a switching power supply system that is provided so that an internal power supply voltage, required for the operation of a control circuit carrying out the switching operation of the switching device, is obtained from an auxiliary winding of the insulating transformer.
2. Background Art
FIG. 3 is a diagram schematically showing the whole configuration of a common flyback switching power supply system. The switching power supply system is provided with a circuit rectifying and smoothing an AC voltage supplied from an AC power supply through a rectifier circuit of a diode bridge DB and an input capacitor Cin, an insulating transformer T to which a rectified and smoothed input voltage Vin is applied, a switching device Q of a MOSFET, for example, connected in series to the primary winding T1 of the insulating transformer T, and a control circuit IC driving the switching of the switching device Q. By carrying out the switching of the switching device Q, a voltage is produced in the secondary winding T2 of the insulating transformer T and the produced voltage is rectified by an output diode Dout. The rectified voltage is then smoothed by an output capacitor Cout, by which a specified output voltage Vout is obtained.
The control circuit IC driving the switching of the switching device Q is provided with a basic function of carrying out PWM control of the switching frequency of the switching device Q to make the output voltage Vout constant according to a signal IS, corresponding to a current Is flowing in the switching device Q and inputted to an IS terminal, and a feedback signal FB showing load information and inputted to an FB terminal. The signal IS inputted to the IS terminal is detected as a voltage across a sense resistor Rs connected in series to the source of the switching device Q. Specifically, the control circuit IC is provided with functions of controlling the turned-on width of the switching device Q according to the value of the current Is and, along with this, controlling the oscillation frequency of the switching device Q according to the value of the feedback signal FB. Moreover, the control circuit IC plays a role of making the output voltage Vout constant by the controlling functions (see JP-A-2011-188586, for example).
The feedback signal FB is obtained as load information indicating the difference between the value of the output voltage Vout, detected by dividing the output voltage Vout with the series connection of resistors Ra and Rb connected in parallel to a load, and the value of a predetermined output voltage in a shunt regulator SR, for example, connected in parallel to the load.
An increase in the output voltage Vout due to a light load increases a current flowing in a light emitting diode in a photocoupler PC connected in series to the shunt regulator SR to increase an amount of the light emission of the light emitting diode. This increases an amount of the light received by a phototransistor in the photocoupler PC to decrease the on-state resistance thereof, by which the voltage of the feedback signal (load information) FB as the output voltage of the phototransistor becomes lower. Conversely, a decrease in the output voltage Vout due to a heavy load decreases a current flowing in the light emitting diode to decrease an amount of the light emission of the light emitting diode. This decreases an amount of the light received by the phototransistor to increase the on-state resistance thereof, by which the voltage of the feedback signal (load information) FB becomes higher. That is, the feedback signal (load information) FB has a voltage signal that becomes lower when a load is light and becomes higher when the load is heavy and is given to the FB terminal of the control circuit IC from the photocoupler PC.
When the load becomes light and the voltage level of the feedback signal FB is lower, the control circuit IC operates to lower the switching frequency of the switching device Q so as to decrease a power loss in the switching power supply system. The control circuit IC is provided with a function of turning-off the switching device Q to stop the switching operation thereof when the feedback signal is below the predetermined lower limit value.
While, the control circuit IC is formed so as to be provided with a start-up circuit for starting an operation by receiving power inputted from the AC power supply AC to a VH terminal through, for example, a diode D1 and a current limiting resistor R1, and at a stable operation after starting, to be operated with a voltage produced in an auxiliary winding T3 of the insulating transformer T used as a driving power supply voltage. Specifically, the control circuit IC is formed so as to be operated by inputting a winding voltage, produced in the auxiliary winding T3 of the insulating transformer T by the switching operation of the switching device Q, through a rectifying and smoothing circuit formed of a diode D2 and a capacitor C and by producing an internal power supply voltage Vcc from the inputted winding voltage (see “10. Application circuit example”, Green Mode PWMIC FA5546/47 (2 Stage OLP Type) Application Note, Fuji Electric Switching Power Supply Control IC, Fuji Electric Co. Ltd., April-2011, English Edition, p. 26, for example).
Moreover, the control circuit IC is provided with an under voltage lock out circuit which stops the operation of the control circuit IC for protecting malfunction thereof when the internal power supply voltage Vcc becomes lower than a predetermined guaranteed operation voltage.